1. Field of the Invention
The present invention relates in general to Satellite Positioning Systems (SATPS), and in particular to a method and apparatus for estimating the Local Oscillator (LO) frequency for Global Positioning System (GPS) receivers.
2. Description of the Related Art
Cellular telephony, including Personal Communication System (PCS) and Personal Data Assistant (PDA) devices, has become commonplace. The use of such devices to provide voice, data, and other services, such as internet access, has provided many conveniences to cellular system users. A current thrust in the cellular and PCS arena is the integration of GPS technology, which is a subset of SATPS technology, into cellular telephone devices and other wireless transceivers.
For example, U.S. Pat. No. 5,841,396, issued to Krasner, which is incorporated by reference herein, describes a method of integrating GPS into wireless devices where the local oscillator for the GPS system is locked to a precision local oscillator frequency that is generated at a basestation of the cellular system. Such an approach, however, requires the cellular system to constantly deliver such a precision frequency to the cellular phone for the integrated GPS system to work properly. Many cellular systems, such as GSM and AMPS, do not have such capability. Further, the Code-Division Multiple Access (CDMA) system that is used for cellular telephony in the United States does not always have a constant delivery for such a frequency; instead, the frequency is delivered in bursts periodically to the cellular telephones. As such, the system described in the Krasner patent would not be of much use.
Further, the Krasner approach requires additional hardware to be installed in the cellular telephone, requiring additional power and weight in a handheld device. Such an approach, where size, power consumption, and weight are such critical parameters, may not be acceptable in the marketplace.
This current interest in integrating GPS with cellular telephony stems from a new Federal Communications Commission (FCC) requirement that cellular telephones be locatable within 20 feet once an emergency call, such as a “911” call (also referred to as Enhanced 911 or “E911”) is placed by a given cellular telephone. Such position data assists police, paramedics, and other law enforcement and public service personnel, as well as other agencies that may need or have legal rights to determine the cellular telephone's position. Further, GPS and/or SATPS data can be used by the cellular user for directions, location of other locations that the cellular user is trying to locate, determination of relative location of the cellular user to other landmarks, directions for the cellular user via internet maps or other GPS/SATPS mapping techniques, etc. Such data can be of use for other than E911 calls, and would be very useful for cellular and PCS subscribers.
It can be seen, then, that there is a need in the art for a method and apparatus for integrating GPS and cellular devices. It can also be seen that there is a need in the art for integrating GPS and cellular devices in such a manner that minimal additional power is used. It can also be seen that there is a need in the art for integrating GPS and cellular devices in such a manner that minimal changes are made to the size and weight of cellular devices. It can also be seen that there is a need in the art for integrated GPS and cellular devices that can work in various cellular systems, e.g., CDMA, GSM, AMPS, etc.